The availability of large quantities of natural gas has led to numerous proposals to upgrade the alkanes in the gas to more valuable chemicals including oxygen-containing organic compounds. The only commercial processes so far developed have required the sequential steps of catalytic dehydrogenation of the alkane to form an olefin followed by oxidation of the reactive olefinic site to produce the oxygenate. Other processes have been proposed for the direct oxidation of alkanes to oxygenates. However, these processes have low conversion and low selectivity and usually require high temperatures.
The two stage vapour phase oxidation of propylene to produce acrylic acid is known in the art. However, the production of acrylic acid from propane would be more attractive than its production from propylene because of the significant price difference between propane and propylene.
There are few references reported in the literature relating to the production of acrylic acid from propane. U.S. Pat. No. 5,198,580 (Standard Oil) discloses a process for partial oxidation of propane to yield acrylic acid, propylene, acrolein, acetic acid and carbon oxides. The process involves the reaction of propane in admixture with a molecular oxygen-containing gas in a reaction zone with a catalyst containing Bib, Moc, Vv, Aa, Dd, Ee, Ox; where A is one or more of K, Na, Li, Cs and TI; D is one or more of Fe, Ni, Co, Zn, Ce and La; E is one or more of W, Nb, Sb, Sn, P, Cu, Pb, B, Mg, Ca and Sr; a, d and e is each from 0 to 10; b is from 0.1 to 10; c is from 0.1 to 20; v is from 0.1 to 10; c:b is from 2:1 to 30:1 and v:b is from 1.5:1 to 8:1. The acrylic acid yield achieved using the bismuth molybdate type of catalyst at a pressure of 138 kPag (20 psig) and a temperature of 400° C. is 5.4% at 19% conversion of propane.
EP-A-0608838 (Takashi et al/Mitsubishi) discloses a method of producing an unsaturated carboxylic acid, mostly in the explosive regime of the propane, air and water mixture, at 380° C. in the presence of a catalyst containing a mixed metal oxide of MoVTeXO, wherein X is at least one element selected from bismuth, cerium, indium, tantalum, niobium, aluminum, boron, tungsten, titanium, zirconium, chromium, manganese, iron, ruthenium, cobalt, rhodium, nickel, palladium, platinum and antimony. The proportion of the respective essential components is based on the total amount of the essential components exclusive of oxygen and satisfies the following formulae: 0.25<rMo<0.98, 0.003<rV<0.5, 0.003<rTe<0.05, 0.003<rX<0.5, wherein rMo, rV, rTe and rX are molar fractions of Mo, V, Te and X, respectively. Takashi et al also disclosed in Patent Number JP1045643 (9845643-February 1998), the formation of acrylic acid and acrolein in the presence of PaMobVcWdXeOn (X=Nb, Ta, Ti, Zr, Sb; if a=1 then b=1−18, c=0−4, d=0−4 and e=0.05−20) at 380° C. achieving a yield of 0.9% to acrolein and 3.5% to acrylic acid at 12% propane conversion
U.S. Pat. No. 6,646,158 (SABIC) suggests the use of a catalyst with a calcined composition of Moa, Vb, Gac, Pdd, Nbe, Xf, wherein X=at least one element selected from the group consisting of La, Te, Ge, Zn, Si, In and W; a is 1; b is 0.01 to 0.9; c is >0 to 0.2; d is 0.000000001 to 0.2; e is >0 to 0.2; and f is >0 to 0.5, for the oxidation of propane to acrylic acid and acrolein. The numerical values of a, b, c, d, e and f represent the relative gram-atom ratios of the elements Mo, V, Ga, Pd, Nb and X respectively in the catalyst. The elements are preferably present in combination with oxygen in the form of various oxides.
The above referenced catalysts disclosed in the literature result in low yields of acrylic acid at relatively high temperatures and most produce propylene as one of the significant by-products. Propylene can be expensive to separate, especially in a recycling mode of operation.
Further examples are taught in the art for the mixed metal oxide component of catalysts for the production of acrylic acid in one step by subjecting propane to a vapour phase catalytic oxidation reaction. Such publications are a Mo—Sb—P—O type catalyst (EP-A-0010902 [Rohm and Haas]); a V—P—Te—O type catalyst (Journal of Catalysis, Col 101, p 389 (1986), a Bi—Mo—O type catalyst and a V—P—Te—O type catalyst (Japanese Unexamined Patent Publication No. 170445/1991). On the other hand, as an example of a catalyst for the production of methacrylic acid in one step by subjecting isobutene to a vapour phase catalytic oxidation reaction, a P—Mo—O type catalyst (Japanese Unexamined Patent Publication No. 145249/1988) is known.
However, each of the methods using such catalysts has a drawback, for example that the yield of the desired unsaturated carboxylic acid is not adequate or the reaction system is complex.
We have now developed a process that enables alkanes to be oxidised under less severe conditions and with a greater selectivity.
Manganese oxides having tunnel (4×4) structure are known and have been proposed as oxidation catalysts, for example in EP-A-0710622 (Texaco). An article in Catalysis Today 85 (2003) pages 225-233 describes the selective oxidation of alcohols using octahedral molecular sieves (OMS). These tunnel shaped manganese oxides contain significant quantities of lattice oxygen. It has been found that this lattice oxygen can be activated to provide available oxygen which can be used for oxidation of organic compounds such as benzyl alcohol to benzaldehyde, 2-butane and cyclohexane, as is described in Catalysis Today 85 (2003) pages 225-233.
It is also known, for example from U.S. Pat. No. 5,597,944 (Texaco) that a transition metal cation can be substituted in the framework of OMS-3 by co-dissolving a transition metal salt in the organic solvent used to dissolve the manganese salt. These materials are proposed as catalysts for the dehydrogenation of n-paraffins to n-olefins. The transition metal cation(s), which can be designated as M<+n> (where M indicates the transition metal and n indicates an oxidation state which is stable in the organic solvent solution), can be any metal selected from Groups IIIA, IVA, VA, VIA, VIIA, VIIIA, IB, IIB and VIIB of the Periodic Table of the Elements (Merck Catalogue of 2001 page 1287). Preferably, the transition metal is a metal selected from Groups IB, IIB and VIII of the Periodic Table of the Elements. Examples of useful framework-substituting transition metals are said to include Mg, Fe, Co, Ni, Cu, Ti, V, Cd, Mo, W, Cr, Zn, La, Ir, Rh, Pd and Pt. Preferred metals include Co, Cu, Ni, Zn, La and Pd. Transition metal cation(s) M<+n> should be present in the organic solvent in a concentration effective to introduce the desired proportions of the transition metal(s) into the framework of OMS-3 structure during the course of the reaction. Therefore, any suitable salt (inorganic or organic) of the selected transition metal(s) can be used which is sufficiently soluble provided, of course, that the anion does not interfere with the other reactants or the course of the reaction. For example, the nitrates, sulfates, perchlorates, alkoxides and acetates can be used with generally good results.
Manganese oxide octahedral molecular sieves possessing mono-directional tunnel structures constitute a family of molecular sieves wherein chains of MnO6 octahedra share edges to form tunnel structures of varying sizes. Such materials have been detected in samples of terrestrial origin and are also found in manganese nodules recovered from the ocean floor. Manganese nodules have been described as useful catalysts in the oxidation of carbon monoxide, methane and butane (U.S. Pat. No. 3,214,236 [Mobil]), in the reduction of nitric oxide with ammonia (Atmospheric Environment, Vol. 6, p. 309 (1972)) and in the demetallation of topped crude in the presence of hydrogen (Ind. Eng. Chem. Proc. Dev., Vol. 13, p. 315 (1974)).
The hollandites are naturally occurring hydrous manganese oxides with tunnel structures (also described as “framework hydrates”) in which Mn can be present as Mn<+4> and other oxidation states. The tunnels can vary in size and configuration and various mono- or di-valent cations can be present in the tunnels. The hollandite structure consists of double chains of MnO6 octahedra which share edges to form (2×2) tunnel structures. The average size of these tunnels is about 4.6 A° square. Ba, K, Na and Pb ions are present in the tunnels and coordinated to the oxygens of the double chains. The identity of the tunnel cations determines the mineral species. Specific hollandite species include hollandite (BaMn8O16), cryptomelane (KMn8O16), manjiroite (NaMn8O16) and coronadite (PbMn8O16). The hydrothermal method of synthesizing a manganese oxide octahedral molecular sieve possessing (2×2) tunnel structures such as those possessed by the naturally-occurring hollandites is described in “Hydrothermal Synthesis of Manganese Oxides with Tunnel Structures,” in Synthesis of Microporous Materials, Vol. II, 333, M. L. Occelli, H. E. Robson Eds. Van Nostrand Reinhold, N.Y., 1992. Such synthetic octahedral molecular sieves having (2×2) tunnel structures are referred to in the art by the designation OMS-2.
The hydrothermal method of producing OMS-2 involves autoclaving an aqueous solution of manganese cation and permanganate anion under acidic conditions, i.e., pH<3, at temperatures ranging from about 80 to about 140° C. in the presence of counter cations having ionic diameters of between 2.3 and 4.6 A°. The counter cations can serve as templates for the formation of OMS-2 product and be retained in the tunnel structures thereof. Based on analytical tests, OMS-2 produced via this method is thermally stable up to 600° C. Alternatively, OMS-2 can be produced by the method disclosed in R. Giovanili and B. Balmer, Chimica, 35 (1981) 53. Thus, when manganese cation and permanganate anion are reacted under basic conditions, i.e., pH>12, a layered manganese oxide precursor is produced. This precursor is ion exchanged and then calcined at high temperatures, i.e., temperatures generally exceeding 600° C., to form OMS-2 product. Analytical tests indicate that OMS-2 produced via this method is thermally stable up to 800° C. and the average oxidation state of manganese ion is lower.
The todorokites are naturally occurring manganese oxides with (3×3) tunnel structures formed by triple chains of MnO6 edge-sharing octahedra. Todorokites and related species are described by Turner et al. in “Todorokites: A New Family of Naturally Occurring Manganese Oxides”, Science, Vol. 212, pp. 1024-1026 (1981). The authors speculate that since todorokites are often found in deep-sea manganese nodules containing high concentrations of copper and nickel, it is probable that such metals substitute for Mn<+2> in the octahedral framework.
Todorokites have attracted particular interest because of their relatively large tunnel dimension and their cation-exchange behaviour which is similar to that of zeolites (Shen et al., “Manganese Oxide Octahedral Molecular Sieves: Preparation, Characterization, and Applications”, Science, Vol. 260, pp. 511-515 (1993)). The naturally occurring todorokites are poorly crystalline, impure in composition and coexist with other manganese oxide minerals. Results of high resolution transmission electron microscopy (HRTEM) show that todorokite contains random intergrowth material of 3×2, 3×3, 3×4 and 3×5 tunnel structure. Because of their disordered structure, the todorokites exhibit variable and non-reproducible catalytic activity, a drawback which militates against their commercial use.
A method of synthesizing a manganese oxide octahedral molecular sieve possessing (3×3) tunnel structures such as those possessed by the naturally-occurring todorokites is described in U.S. Pat. No. 5,340,562 (Texaco). Such synthetic octahedral molecular sieves having (3×3) tunnel structures are referred to in the art by the designation OMS-1.
OMS-1 can be prepared by reacting manganese cation and permanganate anion under strongly basic conditions to form a layered manganese oxide precursor; thereafter aging the precursor at room temperature for at least 8 hours; ion-exchanging the aged precursor; and then autoclaving the ion-exchanged precursor at from 150 to 180° C. for several days. Analytical tests indicate that OMS-1 produced via this method is thermally stable up to about 500° C.
We have now found that the framework of the 2×2 octahedral manganese compound may be ion-exchanged with metal cations to provide a new material and that this material may be used to oxidise alkanes. Our co-pending U.S. patent application Ser. No. 60/950,008, filed concurrently herewith discloses the use of manganese oxide molecular sieves as catalyst for the oxidation of alkyaromatic compounds to the corresponding hydroperoxides.